1. Field of the Invention
The present invention relates to a technique for detecting an interruption state of a signal light in an optical communication field, and in particular, to a signal light interruption detecting method capable of reliably detecting the input interruption of a signal light while suppressing an influence of a noise light, when the signal light is amplified using an optical amplifier, and an optical amplifier using the same.
2. Description of the Related Art
An optical amplifier is an instrument which amplifies a signal light just as it is, and for example, there is a Raman amplifier using a transmission path fiber as an amplification medium, a rare-earth element doped fiber amplifier using a rare-earth element doped fiber as an amplification medium or the like. In the Raman amplifier described above, a pumping light of high level (generally, several 100 mW or above) is given to the transmission path fiber in order to cause the Raman amplification in the transmission path fiber. Therefore, for example, in the case where a breakage or the like occurs in the transmission path fiber, there is a possibility that the pumping light of high level is output from the broken portion to the outside, to cause a problem in the safety of maintenance worker. Further, in the rare-earth element doped fiber amplifier described above, in the case where an input of signal light is interrupted due to an occurrence of failure or the like on the upstream side, since there is a possibility that an internal gain is increased in a silence state, and accordingly, a signal light of excess power is transiently output when the signal light is restored, an optical transmission apparatus on the downstream side may be damaged. In order to avoid such a situation, it is necessary to perform an interruption detection of the signal light input to the optical amplifier, to stop the supply of pumping light in the Raman amplifier or an optical amplification operation in the rare-earth element doped fiber amplifier according to the detection result, or to perform a so-called shut-down control for suppressing the power of the pumping light to a low level at which no problem occurs.
As a conventional signal light interruption detecting method in an optical communication system, there has been known, for example, a method of detecting the presence or absence of a signal light based on the input light power measured by an input light monitor which is provided in an optical amplifier disposed in a transmission and reception terminal station or a repeater station, or a method of extracting a clock component or the like of a light received by a reception terminal station, to perform an interruption detection according to the reception state (refer to Japanese Laid Open Patent No. 2-104153 and Japanese Laid Open Patent No. 10-229365).
However, the conventional signal light interruption detecting method as described above has the following problems. In the signal light interruption detecting method utilizing the input light monitor of the optical amplifier, in the case where the optical amplifier is, for example, a Raman amplifier, the input light monitor is given with a noise light which is generated due to the Raman scattering of a pumping light supplied to a transmission path fiber, together with a signal light which is Raman amplified in the transmission path fiber on an input side. Even when the signal light is interrupted due to a breakage or the like of the transmission path fiber, if the broken portion is apart from the Raman amplifier by a distance of certain degree, this noise light is generated. Therefore, even if the signal light is not inputted due to the occurrence of breakage or the like of the transmission path fiber, it is impossible to detect the input interruption of the signal light due to the existence of the noise light. This is because a difference between the levels of the signal light and the noise light becomes small in the case where a level of the signal light inputted to the light monitor is relatively low
Further, in the method of extracting the clock component or the like in the reception terminal station to perform the interruption detection, since the interruption detection result is transmitted on the optical communication system to be transmitted to the optical amplifier on the upstream side, so that a shut-down control is performed, there is caused a delay until a state of the optical amplifier is actually controlled after the signal light interruption is detected, resulting in a problem of lack of immediacy. Moreover, in a system for transmitting the signal light of high bit rate exceeding for example 10 Gb/s, a circuit corresponding to a high frequency is required to extract the clock component in the reception terminal station. However, such a circuit is generally expensive, and therefore, the cost required for realizing the signal light interruption detection is increased.